This is supposed to be the date that your book was originally published. If your book has appeared in print or digital form before, enter in the date that it was first published.
Amazon has this to say about dates for books that have been published previously (via the bottom of this page):
If your book was previously published, please ensure that the date entered accurately reflects the date of your book’s initial publication. If you are making significant changes to your book’s content and would like the publication date to reflect the date of the updated publication, you must re-submit your revised content as a new book. Ensuring your book’s publication date is accurate helps us provide clear information for our buying customers.
If your book has never been published before, leave this field blank. Amazon will automatically enter in the date that it was published (usually either the day you submitted the book for publication or the day after).
This field is optional. ISBN stands for International Standard Book Number. It’s like a social security number or passport number for books. If you take a book and flip it over to the back and take a look at the barcode, the ISBN will be above the barcode. Like this (ISBN highlighted):
Each book has a unique ISBN, as does each digital version of a book. Print and digital books need different ISBNs.
Your Kindle ebook does not need an ISBN. If you would like one, you can buy one (or many) here.
If you want to submit your Kindle book to different bookstores in the future, note that each version requires a different ISBN. So the Kindle version has one ISBN, the Nook version a different ISBN, the iBookstore version yet another ISBN, and so on.